Diamond-like carbon (DLC) is a dense, partially sp³ bonded form of amorphous carbon prepared by ion beam or plasma deposition. Its unique properties, such as high hardness, low friction, optical transparency, and chemical inertness, make it a valuable coating material. The formation of DLC can be viewed as a phase transition to a denser metastable phase, with the atomic structure consisting of a network of sp³ and sp² sites. The sp³ bonding confers mechanical hardness, while the π states of sp² sites control electronic properties. DLC is deposited using various methods, including ion beam deposition, magnetron sputtering, ion sputtering, laser plasma deposition, and plasma deposition. The deposition process involves the penetration of C⁺ ions into the surface layers, increasing the local density and forming a quenched-in density increase. The sp³ content and density of DLC films can be optimized by controlling the ion energy and deposition conditions. The properties of DLC depend on the deposition process and the sp³ and H content, which can be visualized using a ternary phase diagram. Ion beam deposition, particularly filtered ion beam deposition, can produce highly tetrahedral a-C (ta-C) with up to 85% sp³ content. The sp³ content, density, and hardness of ta-C peak at an optimum ion energy of around 140 eV. Plasma deposition (PD) of a-C:H is also effective, with the properties depending on bias voltage and source gas. The creation of DLC can be described as a phase transition to a denser phase, similar to the transition from graphite to diamond. The intrinsic stress in DLC films, arising from the deposition process, is a key signature of its properties. The stress and density of ta-C:H vary linearly with sp³ fraction, and the high sp³ fraction of ta-C:H is attributed to the high ionization of the plasma beam and the mono-energetic nature of the ions. The electronic structure of DLC is complex, involving both σ and π bonding. The π states of sp² sites form a half-filled band, and their arrangement can open gaps at the Fermi level, affecting the band gap. The band gap of DLC is influenced by the presence of aromatic clusters, which are the smallest configurations that account for the observed optical gaps. The mechanical properties of DLC, such as high Young's modulus and hardness, arise from the strong, directional sp³ bonds. The elastic modulus and hardness of DLC can be described in terms of the mean C-C coordination of the sp³ sites, with the modulus varying with ZCC and the hardness proportional to the modulus. Overall, DLC is a versatile material with a wide range of applications due to its unique properties and the availability of various deposition methods.Diamond-like carbon (DLC) is a dense, partially sp³ bonded form of amorphous carbon prepared by ion beam or plasma deposition. Its unique properties, such as high hardness, low friction, optical transparency, and chemical inertness, make it a valuable coating material. The formation of DLC can be viewed as a phase transition to a denser metastable phase, with the atomic structure consisting of a network of sp³ and sp² sites. The sp³ bonding confers mechanical hardness, while the π states of sp² sites control electronic properties. DLC is deposited using various methods, including ion beam deposition, magnetron sputtering, ion sputtering, laser plasma deposition, and plasma deposition. The deposition process involves the penetration of C⁺ ions into the surface layers, increasing the local density and forming a quenched-in density increase. The sp³ content and density of DLC films can be optimized by controlling the ion energy and deposition conditions. The properties of DLC depend on the deposition process and the sp³ and H content, which can be visualized using a ternary phase diagram. Ion beam deposition, particularly filtered ion beam deposition, can produce highly tetrahedral a-C (ta-C) with up to 85% sp³ content. The sp³ content, density, and hardness of ta-C peak at an optimum ion energy of around 140 eV. Plasma deposition (PD) of a-C:H is also effective, with the properties depending on bias voltage and source gas. The creation of DLC can be described as a phase transition to a denser phase, similar to the transition from graphite to diamond. The intrinsic stress in DLC films, arising from the deposition process, is a key signature of its properties. The stress and density of ta-C:H vary linearly with sp³ fraction, and the high sp³ fraction of ta-C:H is attributed to the high ionization of the plasma beam and the mono-energetic nature of the ions. The electronic structure of DLC is complex, involving both σ and π bonding. The π states of sp² sites form a half-filled band, and their arrangement can open gaps at the Fermi level, affecting the band gap. The band gap of DLC is influenced by the presence of aromatic clusters, which are the smallest configurations that account for the observed optical gaps. The mechanical properties of DLC, such as high Young's modulus and hardness, arise from the strong, directional sp³ bonds. The elastic modulus and hardness of DLC can be described in terms of the mean C-C coordination of the sp³ sites, with the modulus varying with ZCC and the hardness proportional to the modulus. Overall, DLC is a versatile material with a wide range of applications due to its unique properties and the availability of various deposition methods.